AC waveforms biasing for bead manipulating chucks

Abstract

AC waveforms biasing of bead transporter chucks and their accumulated charge sensing circuits tailored for low resistivity substrates and beads where if traditional DC quasi-static biasing potentials were used, the bead attraction potentials of the chuck would undergo rapid RC decay and cause the bead transporter chuck to stop working. Methods for selecting AC waveforms are given, including those that maximize the time average of the bead attraction potential at the bead collection zone of the bead contact surface.

What is claimed is:  
     
       1. An electrostatic chuck for attracting charged grains to a bead collection zone on a bead contact surface comprising: 
       a bead electrode for selectively establishing a grain attracting field to the bead collection zone, the bead electrode shaped and configured in such a manner so that when an AC waveform potential is applied thereto, the grains are influenced by it and guided to selective retention by the bead electrode to the bead collection zone; and  
       an electronic driver programmed to deliver the AC waveform potential configured to provide a repeated effective grain attraction potential at the bead collection zone, wherein the AC waveform potential comprises a grain-attracting potential pulse followed by a recovery period wherein  
       (a) the recovery period is adapted to allow, in the case where the bead collection zone is formed on a substrate with a resistivity below 10 11  Ω-cm, an effective grain attraction field at the bead collection zone on activation of a subsequent grain-attracting potential pulse, wherein the AC waveform potential has a frequency of 100 Hz or less, or  
       (b) the AC waveform potential corresponds to applied voltages and time periods effective so that the voltage at the bead collection zone resulting from the grain-attracting potentials following a grain-attracting potential (other than an initial grain-attracting potential) is at least 10% of an initial voltage at the bead collection zone, or  
       (c) the AC waveform potential corresponds to applied voltages and time periods effective so that the integrated voltage at the bead collection zone integrated over the pulse period (other than an initial pulse period) during which a grain-attracting voltage is applied voltage at the bead collection zone is at least 20% of an initial integrated voltage.  
     
     
       2. The electrostatic chuck of claim  1 , wherein the electronic driver is programmed to deliver the AC waveform potential configured such that the grain attraction potential on the bead collection zone is greater, on average, than that obtained when applying a time-averaged DC potential corresponding to the AC waveform potential. 
     
     
       3. A method for adhering charged grains to a bead collection zone on a bead contact surface comprising: 
       (1) adhering a substrate with a resistivity below 10 11  Ω-cm to an electrostatic chuck, with the substrate arrayed over a grain-attracting electrode of the electrostatic chuck so that a surface of the substrate defines the bead contact surface;  
       (2) applying an AC waveform potential to the grain-attracting electrode to create a grain attracting field, wherein the AC waveform potential comprises a grain-attracting potential pulse followed by a recovery period adapted to allow an effective grain attraction potential at the bead collection zone on activation of a subsequent grain-attracting potential pulse,  
       wherein the AC waveform potential has a frequency of 100 Hz or less, or  
       the AC waveform potential corresponds to applied voltages and time periods effective so that the voltage at the bead collection zone resulting from the grain-attracting potentials following a grain-attracting potential is at least 10% of an initial voltage at the bead collection zone, or  
       the AC waveform potential corresponds to applied voltages and time periods effective so that the integrated voltage at the bead collection zone integrated over the pulse period during which a grain-attracting voltage is applied voltage at the bead collection zone is at least 20% of an initial integrated voltage; and  
       (3) attracting and retaining a grain to the bead collection zone.  
     
     
       4. The method of claim  3 , additionally comprising: 
       (5) providing an accumulated charge sensing circuit for the bead transporter chuck having a charge sensing electrode for monitoring accumulated charge on a bead collection zone of the bead contact surface, comprising  
       (a) an AC bias source, (2) a sensing capacitor electrically connected between the charge collector electrode and the AC bias source, and  
       (b) an electrometer electrically connected between the AC bias source and the coupling capacitor to measure the potential the sensing capacitor;  
       (6) terminating the application of the AC waveform potential when the charge sensing circuit indicates that sufficient grains have been accumulated at the bead collection zone, or, where there are multiple bead collection zones and two or more charge sensing circuits, adjusting AC waveform potentials so that grain accumulation at various bead collection zones is adjusted.  
     
     
       5. The method of claim  3 , wherein the AC waveform potential is effective to apply at least 300 μg/4 mm of charged powder to a said bead collection zone. 
     
     
       6. The method of claim  3 , wherein the AC waveform potential is configured such that the grain attraction potential at thc bead collection zone is greater, on average, than that obtained with a time-averaged DC potential corresponding to the AC waveform potential. 
     
     
       7. The method of claim  3 , comprising fabricating dosage forms by: 
       providing an edible substrate as the substrate; and  
       operating the electrostatic chuck pursuant to steps (1) through (3) to deposit measured amounts of powder, comprising a pharmaceutically active substance, on spatially resolved regions of the edible substrate.  
     
     
       8. The method of claim  3 , wherein the recovery period is at least 1.0 times substrate resistivity (in Ω-cm) times 10 −9  mS.